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NETWORK WORKING GROUP L. Zhu
Internet-Draft Microsoft Corporation
Updates: 4120 (if approved) October 17, 2006
Intended status: Standards Track
Expires: April 20, 2007
Kerberos for Web Services
draft-zhu-ws-kerb-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 20, 2007.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines extensions to the Kerberos protocol and the
GSS-API Kerberos mechanism that enable a GSS-API Kerberos client to
exchange messages with the KDC using the GSS-API server as the proxy,
by encapsulating the Kerberos messages inside GSS-API tokens. With
these extensions, Kerberos is suitable for securing communication
between the client and web services over the Internet.
Zhu Expires April 20, 2007 [Page 1]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . . 3
3. GSS-API Encapsulation . . . . . . . . . . . . . . . . . . . . . 3
4. Addresses in Tickets . . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
8. Normative References . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 9
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1. Introduction
The Kerberos [RFC4120] pre-authentication framework [KRB-PAFW]
promises minimal or no exposure of weak client keys and strong client
authentication. The Kerberos support of anonymity [KRB-ANON]
provides for privacy. These advancements make Kerberos suitable over
the Internet.
The traditional client-push Kerberos protocol does not work well in
the Web services environments where the KDC is not accessible to the
client, but is accessible to the web server. For example, the KDC is
commonly placed behind a firewall together with the application
server. The lack of accessibility to the KDC by the client could
also occur are when the client does not have an IP address prior to
authenticating to an access point.
Generic Security Service Application Program Interface (GSS-API)
[RFC2743] allows security mechanisms exchange arbitrary messages
between the client and the server via the application traffic,
independent of the underlying transports. A protocol based on
[RFC4121] is thus defined to allow the GSS-API client to exchange
Kerberos messages with the KDC by using the GSS-API server as the
proxy. The server-pull model defined in this specification is
benefical for clients with limited processing power such as mobile
devices, or when the client and the server message exchange has high
network latency.
Client <---------> WS-KERB proxy <----------> KDC
The Kerberos client MUST use a pre-authentication mechanism such as
FAST [KRB-PAFW] to avoid exposure of long term client keys to the
server, before and after the server is authenticated, and hide the
client identity from adversary who can eavesdrop the application
traffic if such level of privacy is desirable.
2. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. GSS-API Encapsulation
The mechanism Objection Identifier (OID) for GSS-API WS-KERB, in
accordance with the mechanism proposed by [RFC4178] for negotiating
protocol variations, is id-kerberos-ws:
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id-kerberos-ws ::=
{ iso(1) org(3) dod(6) internet(1) security(5) kerberosV5(2)
webService(6) }
The first token of the GSS-API WS-KERB mechanism MUST have the
generic token framing described in section 3.1 of [RFC2743] with the
mechanism OID being id-kerberos-ws, and any subsequent GSS-API WS-
KERB token MUST NOT have this framing.
The GSS-API WS-KERB mechanism MUST always provide server
authentication, even if the client does not ask for it. When server-
authentication is performed, the GSS-API server will always send back
an AP-REP, and as described later in this section this mechanism
provides integrity protection for all client-server proxy message
exchanges.
The innerToken described in section 3.1 of [RFC2743] and subsequent
GSS-API tokens have the following formats: it starts with a two-octet
token-identifier (TOK_ID), followed by a WS-KERB message or a
Kerberos message.
Token/Message TOK_ID Value in Hex
-----------------------------------------
WS_KRB_PROXY 05 01
Only one WS-KERB specific message, namely the WS_KRB_PROXY message,
is defined in this document. The TOK_ID values for [RFC4120]
Kerberos messages are the same as those defined for the GSS-API
Kerberos mechanism [RFC4121].
The message of the WS_KRB_PROXY type is defined as a WS-KRB-HEADER
structure immediately followed by a Kerberos message. The Kerberos
message can be an AS-REQ, an AS-REP, a TGS-REQ, a TGS-REP, or a KRB-
ERROR as defined in [RFC4120].
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WS-KRB-HEADER ::= SEQUENCE {
pvno [1] INTEGER (5) ,
msg-type [2] INTEGER (23),
proxy-data [3] ProxyData,
...
}
ProxyData :: = SEQUENCE {
realm [1] Realm,
cookie [3] OCTET STRING OPTIONAL,
-- opaque data, if sent by the server,
-- MUST be copied by the client unchanged into
-- the next WS-KERB message.
...
}
The WS-KRB-HEADER structure and all the Kerberos messages MUST be
encoded using Abstract Syntax Notation One (ASN.1) Distinguished
Encoding Rules (DER) [X680] [X690].
The GSS-API WS-KERB client fills out the realm field in the ProxyData
of the first request with the client realm. If the client does not
know the client realm [REFERALS], it MUST fill it out using the
client's default realm name such as the realm of the client host.
Typically the Kerberos message in the first WS_KRB_PROXY message from
the client is an AS-REQ message.
Upon receipt of the WS_KRB_PROXY message, the GSS-API WS-KERB server
MUST use the proxy-data in the message from the client to locate the
KDC and sends the encapsulated Kerberos message to that KDC. In
order to reduce the number of roundtrips between the client and the
server, the server SHOULD process any message exchange with the KDC
if the exchange is unauthenticated, such as client-referral message
exchange [REFERALS]. If the server can not process the KDC response
by itself, for example when the knowledge of the client keys is
required, it sends back to the client the KDC reply message
encapsulated in a WS_KRB_PROXY message. The server MUST fill out the
realm name whose KDC produced the response. The server SHOULD use
the XKDC mechanism described in [KRB-PAFW] to allow the client's KDC
to obtain a service ticket to the server, thus further reduce the
number of roundtrips between the GSS-API client and the GSS-API
server. The GSS-API server can send opaque data in the cookie field
of the WS-KRB-HEADER structure in the server reply to the client, in
order to, for example, reduce the amount of state information kept by
the GSS-API server. The content and the encoding of the cookie field
is a local matter of the server. The client MUST copy the verbatim
cookie from the previous server response, when the cookie is present,
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whenever it sends an additional WS-KRB-PROXY message to the server.
The client processes the KDC response, and fills out the realm name
it believes to whom the server should send the encapsulated Kerberos
message.
When the client obtains a service ticket, the client then sends a
KRB_AP_REQ message to the server, and proceed as defined in
[RFC4121]. A GSS-API authenticator extension [GSS-EXTS] MUST be sent
by the client. The extension type is 2 and the content is the ASN.1
DER encoding of the type Checksum. The checksum is performed over
all GSS-API messages as exchanged between the client and the server
before the KRB_AP_REQ message, and in the order of the exchange. The
checksum type is the required checksum type for the enctype of the
subkey in the authenticator, the protocol key is the authenticator
subkey, and the key usage number is TBA-1. The server MUST verify
this checksum in the GSS-API authenticator extension, then respond
with an AP-REP extension [GSS-EXTS]. The AP-REP extension type is 2
and the the content is the ASN.1 DER encoding of the type Checksum.
This checksum is performed over all GSS-API messages as exchanged
between the client and the server before the KRB_AP_REQ message, and
in the order of the exchange. The checksum type is the required
checksum type for the enctype of the authenticator subkey in the
request, and the protocol key is the authenticator subkey, and the
key usage number is TBA-2. The client MUST then verify this
checksum.
4. Addresses in Tickets
In WS-KERB, the machine sending requests to the KDC is the GSS-API
server and not the client. As a result, the client should not
include its addresses in any KDC requests for two reasons. First,
the KDC may reject the forwarded request as being from the wrong
client. Second, in the case of initial authentication for a dial-up
client, the client machine may not yet possess a network address.
Hence, as allowed by [RFC4120], the addresses field of the AS-REQ and
TGS-REQ requests SHOULD be blank and the caddr field of the ticket
SHOULD similarly be left blank.
5. Security Considerations
Similar to other network access protocols, WS-KERB allows an
unauthenticated client (possibly outside the security perimeter of an
organization) to send messages that are proxied to interior servers.
In a scenario where DNS SRV RR's are being used to locate the KDC,
WS-KERB is being used, and an external attacker can modify DNS
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responses to the WS-KERB proxy, there are several countermeasures to
prevent arbitrary messages from being sent to internal servers:
1. KDC port numbers can be statically configured on the WS-KERB
proxy. In this case, the messages will always be sent to KDC's.
For an organization that runs KDC's on a static port (usually
port 88) and does not run any other servers on the same port,
this countermeasure would be easy to administer and should be
effective.
2. The proxy can do application level sanity checking and filtering.
This countermeasure should eliminate many of the above attacks.
3. DNS security can be deployed. This countermeasure is probably
overkill for this particular problem, but if an organization has
already deployed DNS security for other reasons, then it might
make sense to leverage it here. Note that Kerberos could be used
to protect the DNS exchanges. The initial DNS SRV KDC lookup by
the proxy will be unprotected, but an attack here is at most a
denial of service (the initial lookup will be for the proxy's KDC
to facilitate Kerberos protection of subsequent DNS exchanges
between itself and the DNS server).
6. Acknowledgements
The server-proxy idea is based on the early revisions of this
document written by Jonathan Trostle, Michael Swift, Bernard Aboba
and Glen Zorn.
The rest of the ideas mostly came from hallway conversations between
the author and Nicolas Williams.
7. IANA Considerations
There is no IANA action required for this document.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2743] Linn, J., "Generic Security Service Application Program
Interface Version 2, Update 1", RFC 2743, January 2000.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Zhu Expires April 20, 2007 [Page 7]
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Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC4121] Zhu, L., Jaganathan, K., and S. Hartman, "The Kerberos
Version 5 Generic Security Service Application Program
Interface (GSS-API) Mechanism: Version 2", RFC 4121,
July 2005.
[RFC4178] Zhu, L., Leach, P., Jaganathan, K., and W. Ingersoll, "The
Simple and Protected Generic Security Service Application
Program Interface (GSS-API) Negotiation Mechanism",
RFC 4178, October 2005.
[KRB-ANON] Zhu, L., Leach, P. and Jaganathan, K., "Kerberos Anonymity
Support", draft-ietf-krb-wg-anon, work in progress.
[KRB-PAFW] Zhu, etl, "Kerberos Pre-Authentication framework",
draft-ietf-krb-wg-preauth-framework, work in progress.
[GSS-EXTS] Emery, S., draft-ietf-krb-wg-gss-cb-hash-agility, work in
progess.
[REFERALS] Raeburn, K., "Generating KDC Referrals to Locate Kerberos
Realms", draft-ietf-krb-wg-kerberos-referrals, work in
progress.
[X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
Information technology - Abstract Syntax Notation One
(ASN.1): Specification of basic notation.
[X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER).
Author's Address
Larry Zhu
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
US
Email: lzhu@microsoft.com
Zhu Expires April 20, 2007 [Page 8]
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
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Acknowledgment
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Zhu Expires April 20, 2007 [Page 9]

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2006-10-19 Love H<>rnquist <20>strand <lha@it.su.se>
* Makefile.am: Add more files.
2006-10-17 Love H<>rnquist <20>strand <lha@it.su.se>
* ks_file.c: set ret, remember to free ivdata